Added the useful thinking bit of the autoupdate and tests.

pre_commit/commands.py

@@ -4,6 +4,10 @@ from __future__ import print_function
 import os
 import pkg_resources
 import stat
+from plumbum import local
+
+from pre_commit.ordereddict import OrderedDict
+from pre_commit.repository import Repository
 
 
 def install(runner):
@@ -29,3 +33,46 @@ def uninstall(runner):
         os.remove(runner.pre_commit_path)
         print('pre-commit uninstalled')
     return 0
+
+
+class RepositoryCannotBeUpdatedError(RuntimeError): pass
+
+
+def _update_repository(repo_config):
+    """Updates a repository to the tip of `master`.  If the repository cannot
+    be updated because a hook that is configured does not exist in `master`,
+    this raises a RepositoryCannotBeUpdatedError
+
+    Args:
+        repo_config - A config for a repository
+    """
+    repo = Repository(repo_config)
+
+    with repo.in_checkout():
+        local['git']['fetch']()
+        head_sha = local['git']['rev-parse', 'origin/master']().strip()
+
+    # Don't bother trying to update if our sha is the same
+    if head_sha == repo_config['sha']:
+        return repo_config
+
+    # Construct a new config with the head sha
+    new_config = OrderedDict(repo_config)
+    new_config['sha'] = head_sha
+    new_repo = Repository(new_config)
+
+    # See if any of our hooks were deleted with the new commits
+    hooks = set(repo.hooks.keys())
+    hooks_missing = hooks - (hooks & set(new_repo.manifest.keys()))
+    if hooks_missing:
+        raise RepositoryCannotBeUpdatedError(
+            'Cannot update because the tip of master is missing these hooks:\n'
+            '{0}'.format(', '.join(sorted(hooks_missing)))
+        )
+
+    return new_config
+
+
+def autoupdate(runner):
+    """Auto-update the pre-commit config to the latest versions of repos."""
+    pass